Reloading problem with the K-31

[tobnpr]

Posting this on a few forums I frequent, to see if anyone can offer an explanation for this...


So...
Decided to handload some rounds for my son's K-31 last trip to the range.

Didn't have the time to do the usual "due diligence"...so I just went by the Sierra manual first time out.

With 46.5 of H4350 under the 175 SMK, we were getting about 2390 fps and tight groups. No pressure signs, but he was having a LOT of trouble closing the bolt fully. Got about 15 rounds off, then decided to call it quits, rather than risking the bolt not being fully in battery until I could figure out the problem.

So...
Getting ready to head back to Manatee this week. I check the OAL of the rounds I loaded for him. Spot on, at 3.06" OAL with the 175 SMK per the Sierra manual....

Now, I never load bullets by OAL, I always use a comparator and load off the lands. So, I grab the OAL gauge, and the modified case that I had since ordered, and chucked in a 175 SMK.

Seated the case in the chamber, tapped the bullet into the lands, locked it down and withdrew it.

Holy chit...

It didn't take calipers to see what the problem was. I mean, I've NEVER seen anything like it. Half the bullet was in the case...

So here's the numbers...

At an OAL of 3.06, the comparator gives me a number of 2.484...
Touching the rifling, the comparator gives me 2.277.

At the OAL specified in the manual, I was jamming the bullet into the rifling by .207...

The OAL (not comparator length) of the bullet touching the lands is 2.845- one hell of a far cry from 3.06...


Somethin' isn't right here...

Either, Sierra's manual is useless, or this K-31 has an out-of-spec chamber?

Now, it's obvious that we can't jam two-tenths of an inch of bullet into the rifling, thank God we didn't blow anything up...and yeah, should have stopped even before we got fifteen rounds off. But, I thought it was likely a sticky bolt issue, since the cases weren't showing any pressure signs and extraction was not difficult.

The 175 SMK is almost identical in weight to the 174 grain bullet used in the GP-11 ammo, and looks to have a similar ogive- though I don't have any at the moment to do a comparison .

Appreciate any input from those of you that might handload this round...

Picture says it all. The one on the left is in the comparator, and touching the rifling. The one on the right- loaded to 3.06" per the Sierra manual...

[diopter]

Most published C.O.L. loading data for the 7.5x55 Swiss comes from the M1911 and M96/11 Swiss rifles which have a longer throat than the K31.

Only Vihtavuori and Norma list the proper C.O.L for the K31 with their bullets.
They refer to the cartridge as GP31 to differentiated it from the GP11 round as used in the longer throat from the older generation rifles. Still same cartridge and specs.

If I may, I'll post some links about the topic.

Posts and Chart for Commonly used bullets and their C.O.L in their K31s
http://theswissriflesdotcommessagebo...x55-in-the-K31

Load Tables
http://theswissriflesdotcommessagebo...55-Load-Tables

[lonniemike]

You can find exceptions but usually most projectiles for K-31's will have an OAL of 2.8 to 2.9" or so range. I need to get down to Manatee also.Best

[tobnpr]

Diopter- thanks...
I posted this same question on that forum as well.

I see from the chart in the link you posted that it lists the 175 SMK at 2.92.
Still a bit longer than the 2.84 I was getting, but sure a lot closer...

I would never have guessed there could be such a huge difference in the throats for the same caliber.

[Unclenick]

Don't forget the K31's typically have a .304"-.306" groove diameter barrels for that slightly smaller 7.5 mm bullet. It's not normally too much to ask a rifle to swage down a .308" to fit, but that's still enough groove diameter difference that the freebore may be only .306"-.308", so your bullet is probably stopping on the mouth of the freebore and not in the throat of the rifling. Hence the big difference in COL.

If I had one of these rifles, I think I would moly-plate the bullets to ease the swaging process. I might then push them through a .304" sizing die so I could seat them out where they belong. Before that, I would slug the bore and the throat and freebore to see what I was actually dealing with.

[Scimmia]

Don't forget the K31's typically have a .304"-.306" groove diameter barrels for that slightly smaller 7.5 mm bullet.

Wait, what? Where did this information come from? I haven't seen anyone say they had a barrel slug out at .304", it's usually .307" and occasionallly .306". According to the Swiss military, GP11 uses a bullet up to .3087".

Edited to correct a misremembered measurement.

[diopter]

"Don't forget the K31's typically have a .304"-.306" groove diameter barrels for that slightly smaller 7.5 mm bullet."

True only for the part of the GP11 bullet that sticks out of the case.
The widest part is right where the boat tail starts.
.3075"
In other words the GP11 does not have a parallel shank, it tapers ALL the way to the boat tail.

7.5mm refers to the bore(avg ±7.54= .296"), same as you would refer a .308" diameter bullet for a .30 caliber bore.

.308" is actually 7.82mm diameter.

[hodaka]

I had similar problems when I first started reloading for my K-31. Now I use trial and error to set the OAL. It varies a lot between bullets. I am not with my data now but 168 SMK's are seated very deep into the case whereas the PRVI 175's can be much longer. As pointed out earlier, the bullet taper is the key. Good luck.

[lonniemike]

diopter has it right. For 31's, I recommend an OAL range of 2.8 to 2.9" or so, until one can find a best length. The 11 types unlike the 31's have a more generous chamber that could accommodate the earlier GP-90/23 ammo that preceded GP-11 ammo. There are still plenty of published facts that just aren't so. Let us add to that what someone believes, has heard of or measured perhaps accurately and lets bring in a little confusion too. The current info is better than past but it can still be a struggle sometimes. Thanks Carlos Best All

[diopter]

GP11 on left & Berger 175gr VLD Target on right.




You see the way the GP11 bullet tapers all the way to the Boat Tail Compared to the Berger and 99.9% of other bullets

[Unclenick]

Bullets often have a pressure ring at the bottom of the bearing surface that's wider. Half a thousandth taper is common. Set the bullet sideways in a pair of caliper jaws and look for the light cracks to see.


Scimmia,

QuickLOAD's drawing (often off by a thousandth) of the cartridge puts the bullet at .304" at the case mouth, but its database puts it at .306". The cartridge drawing on Wikipedia, taken from the CIP drawings, show the bullet at 7.78 mm (.3063"). But as with SAAMI practice, the given diameter of the bullet is a maximum with a minus tolerance. (For example, in .30-06 and .308", SAAMI gives the bullets as 0.309" -.003", but the chamber and barrel as .308" groove and .300" bore.) Thus I would expect the CIP bullet diameter to be on the large side of groove spec.

I ran into a description of the OP's loading issue with two K31's in which the groove diameters slugged 0.305". Then the last post of this thread says the CIP bore drawing dimensions are:

CIP bore diameter = 7.51 mm = 0.2957"
CIP groove diameter = 7.74 mm= 0.3047"

A look online for uploaded CIP drawings at Scribd had one updated in '02 that agreed with the bore diameter above but gave a larger 7.77 mm (.3059") as the groove diameter for test barrels. The drawing looks official but is missing some tolerance information and gives the start of the rifling as a minimum groove diameter of just 7.70 mm (.3031"). So I don't know how far this drawing is to be trusted or if that's just what their tolerances allow?

Given the disagreeing information, but especially because the first linked to post reporting slugged two K31 barrels at 0.305", tending to validate the smaller groove diameter from the second thread link, it is reasonable to believe a rifle might have Ø0.305" groove a roughly Ø0.307" freebore, which would stop a .308 bullet from entering without a lot of pressure. (the Scribd drawing shows a 3 mm (0.118") long freebore, but gives it no diameter dimension). This is why I suggested the OP slug the chamber throat. Alternately, he could simply put magic marker all over his bullet and see what the contact marks looks like. If it looks like rifling marks, then it's just a very short throated barrel. If the mark is a ring, then he's hit the freebore and will need to narrow the bullets to achieve full 3.060" (77.7 mm) COL.

[tobnpr]

What is the spec for the bore/groove dia. for the K-31?
I know there's huge variations with the likes of the Mosin-Nagant, but we're talking Swiss watches here, aren't we?

If they have a groove dia. of .306, I'm not overly concerned about .308 bullets down the bore. Everything I've read says .308 bullets are fine for this rifle...
But I do wonder whether I can achieve the same/similar velocity of the GP-11 (2560) without pressure issues due to the slightly oversize bullet?

The 175 SMK would seem to take up a bit more case capacity since it doesn't have a rebated boattail like the GP-11.

I wonder if it's even worthwhile reloading for this rifle, since the GP-11 is so accurate, and cost is similar to rolling my own match rounds.

[Scimmia]

taken from the CIP drawings

Do these drawings mean anything, though? If Switzerland manufacturered to CIP specs, maybe, but the Swiss are not members of C.I.P. This is their own cartridge and rifles manufactured to their own specification, C.I.P has nothing to do with it.

I do appreciate the link to the K31s slugging out at .305". Like I said, almost all of them I have seen posted are .307" with some .306".

[Unclenick]

I just don't know what was official from all the different numbers. My assumption was the CIP borrowed the military specs (why make up a different spec) as they do with their drawings of the NATO rounds. But it's really the existence of those slugged .305's that made me think the guy who said the groove diameter was .3047" might be right or might have been right about one point in its history. They only made about half a million, I think, but I don't know if the design changed at any during production. At any rate, it seems like stopping a .308 on the freebore is quite possible if you have the narrow bore, assuming the chamber reamers were made to match. I just need a chamber cast or slug to be sure.

Every once in awhile you hear of someone successfully shooting a .308 bullet through a 7 mm rifle without permanent damage. Not something you can count on getting away with, but the smaller difference in the 7.5, however real it is, doesn't matter much by comparison. I wouldn't expect it to be a problem up to 0.005" difference or so.

[diopter]

Swiss rifles Bore Specs:
http://theswissriflesdotcommessagebo...les#reply-4179


Bore Tolerance Specifications for Different Swiss Rifles

G1889-K1893: 7.49mm-7.56mm
1911/K11: 7.54mm-7.60mm shot out at 7.64mm
K31/Zfk55/PE57: 7.51mm-7.56mm shot out at 7.64mm

[g.willikers]

Another reason to treat published data as guides and not universal truths.
And every shootin' iron as unique.
I've had guns that were enough off from the specs that they needed reloading dies for a different caliber, all together.

[lonniemike]

How's this for tolerances?
I recieved in response to some of my questions about Swiss rifles from W+F Bern(Swissgovassemblefactory). At that time I was asking 1911 questions as 1931's were a dream to me. On the W+F barrel data sheet dated 7.10.1992/Szr is a groove diameter given as 7,78 +0.06 mm(actually my keyboard won't do it but the +0.06 is over another zero like +0,06/0 without the / whatever that represents or means) or in inches it shows .3063 +0.024 inch over a similar as above zero. And W+F shows a bullet dia of 7.84 -0.04 mm with a zero over the -0.04 and in inches it shows .3087 -0.016(likewise a zero is above the -0.016.
What does it all mean? 304 thru 306 dia barrels or bullets are real hard to swallow. and I would attribute most if not all to bad measurements and or techs. I remember a long time ago a fellow named Pierre found out a caliper of his was a .001" off for the GP-11 projectile. A mistake that he quickly corrected.
Guisan sometimes posts neat pics of gauges, reamers, and other things showing sometimes interesting manufacturing/tolerance info. Best

[Unclenick]

Lonnie,

You need an extra zero here an there. You're using the centimeter to inch conversion rather than the millimeter to inch conversion which requires you divide mm by 25.4, but your point in principle is still well taken. Lots of disagreement about the specs out there. Be nice just to measure some randomly pulled Swiss army ammo bullets with a micrometer (not a caliper) that's been checked on a standard just to see what their army actually issued on average at the time. Even that will have a tolerance, though.

The divide by sign is just one way to separate the + tolerance and - tolerance. It means the tolerance, in the first instance is + 0.06mm / -0.00 mm, or + 0.0024"/ -0.0000" meaning it can be bigger but not smaller. Very unusual for a bullet. Normally their numbers are given as a maximum with a +0.00 mm/-0.05 mm or -0.08 mm or thereabouts.

[diopter]

1984 GP11 .3076"

[Scimmia]

diopter, what is the measurement of that bullet at the mouth of the case? I think we've covered the max diameter pretty well, as you post confirms, but Unclenick's main point of the freebore being too small really depends on the part of the bullet outside the case.

[diopter]

On this 1984 it is .3029"



Pulled bullet scratched too much by bullet puller to trust measurements.
Pulled bullet was in .2996" to .2999" range

[lonniemike]

Okay who has a barrel's chamber cast with a freebore or groove diameter for a 7.5 Swiss rifle showing a max dia of only.304, 305, 306, or dare I say .307". I think it'll be a short list. Nick if my photobucket Fu works out, you'll be able to see the Swiss paper and explain exactly where here and there needs [/IMG]to be some extra zeros.well fu on me and photobucket.maybe later

[lonniemike]

s97.photobucket.com/albums/l205/lonniemike/?action=view&current=star.jpg

[Scimmia]

diopter, with that measurement sticking out, it's very reasonable that the freebore would be smaller than we're used it.

lonniemike, look at where you put the decimal point for the +/- measurement vs what's on that sheet and I think you'll see what Unclenick was talking about. It's not .3063 +0.024, it's .3063 +.0024

[lonniemike]